Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, alcohol and alcohol-containing fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. Various engine systems may be used with alcohol fuels, utilizing various engine technologies and injection technologies. Further, various approaches may be used to control such alcohol-fuelled engines to take advantage of the charge-cooling effect of the high octane alcohol fuel, in particular to address engine knocking. For example, engine control methods may include adjustment of boost or spark timing in dependence upon the alcohol fuel, and various other engine operating conditions.
Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust to an engine intake. For example, an EGR valve may be actuated to adjust an amount of exhaust gas that is recirculated to the engine intake. While providing a desired engine dilution, such EGR systems may improve engine performance by reducing engine knock, throttling losses, heat transfer losses, as well as NOx emissions.
However, the inventors herein have recognized that EGR transients may be generated during engine operating conditions when a sudden increase or decrease in the amount of EGR desired occurs. Transient control of EGR is difficult, due to the delay between the time of actuation of an EGR valve and the corresponding change in dilution at the cylinder. For example, in response to a sudden tip-in from light load to medium load, an EGR valve may be adjusted to increase an amount of EGR provided. However, until the EGR gases mix with intake air throughout the intake manifold and the amount of EGR at the cylinders reaches the new desired amount, engine efficiency may be degraded. Similarly, in response to a sudden tip-out from medium load to light load, an EGR valve may be adjusted to decrease an amount of EGR provided. However, until the amount of EGR pre-mixed with air in the intake manifold is consumed by the cylinders, the actual EGR level in the cylinders will be higher than desired, and engine combustion stability and efficiency may be degraded.
In one example, the above issues may be at least partly addressed by a method of operating an engine including EGR. In one embodiment, the method comprises, during an increase in EGR flow from a first amount to a second, higher, amount, increasing water injection more rapidly to a first water injection amount and then more slowly decrease water injection amount to a second amount lower than the first amount. In another embodiment, the method comprises, adjusting an engine load at which water is direct injected into an engine cylinder based on the EGR flow.
In one example, an engine may be configured with a turbocharger for providing a boosted air charge, as well as an exhaust gas recirculation (EGR) passage for diverting at least some exhaust gas from the engine exhaust to the engine intake. In one example, the EGR passage may be configured to provide low pressure (LP) EGR wherein the exhaust gas is recirculated from the exhaust downstream of a turbocharger turbine to the intake upstream of a turbocharger compressor. In an alternate embodiment, the EGR passage may be configured to provide high pressure (HP) EGR wherein the exhaust gas is recirculated from the exhaust upstream of the turbocharger turbine to the intake downstream of the turbocharger compressor. The engine may also be configured with a direct injector for direct injecting a knock control fluid into an engine cylinder. In one example, the injected fluid may be water. In alternate examples, the injected fluid may be an alcohol-gasoline fuel blend or an alcohol fuel such as ethanol or methanol, or a mixture of one or more of these fuels with water. Herein, the inherent octane effects and/or the charge cooling effects and/or the dilution effects of the direct injected fluid may be used to address cylinder knock, reduce engine NOx emissions, and/or provide at least some engine dilution.
Based on engine operating conditions, a desired amount of engine dilution may be determined. For example, the desired amount of dilution may be based on an engine speed-load conditions, a likelihood of knock, exhaust temperature, an emission control device temperature, etc. As such, under some engine conditions, the desired dilution may be largely provided by EGR. Thus, based on the amount of dilution desired, an amount of EGR that may provide the desired engine dilution may be determined.
In response to a sudden increase or decrease in the desired amount of dilution, an EGR valve in the EGR passage may be actuated to increase or decrease the provided amount of EGR. Further, to compensate for EGR transients that may arise due to the delay between the actuation of the EGR valve and the change in engine dilution at the cylinder, an amount of knock control fluid, for example, water, direct injected into the cylinder may be increased (for example, from a first amount) to provide the required difference in engine dilution substantially immediately. As such, the amount of water direct injected may be adjusted to reflect an amount required to address engine knock at the prevalent engine operating conditions as well as to provide a dilution to reduce EGR transient control problems. Herein, by direct injecting water into the cylinder, a substantially immediate vaporization of the injected water into vapor may be achieved, thereby providing a faster change in engine dilution as compared to the actuation of the EGR valve.
As the EGR starts to take effect and is ramped in to provide the desired dilution, the water injection may be decreased or ramped out. For example, following the increase in water injection from the first amount, the amount of water injected may be gradually decreased to a second amount. The second amount may reflect an amount of water required to address knock only, once the desired amount of EGR has been ramped in. Thus, in one example, the second amount of water injected may be lower than the first amount.
To address sudden transients that may arise during the ramping in of the EGR, for example, due to a sudden pedal tip-in or tip-out by the engine operator, the rate of decrease in the water injection amount may be adjusted to be slower than the rate of increase in response to the presence of EGR transients. That is, when the desired amount of EGR is not available, the water injection may be rapidly increased to immediately provide the desired dilution. However, even after the desired amount of EGR is available, the water injection may be gradually decreased so that sudden unexpected EGR transients are better addressed.
To address sudden transients that may arise during the ramping out of EGR, for example, due to a sudden pedal tip-in or tip-out by the engine operator, the amount of dilution with EGR may be limited during steady state operation. For example, at medium load the total desired dilution may be high, but achieving this dilution with EGR may not be feasible because it is impossible to quickly reduce the actual EGR at the cylinder in case of a subsequent tip-in or tip-out which may lead to a sudden decrease in desired dilution. Under these conditions, the total desired dilution may be achieved with a combination of EGR plus water injection. If a sudden decrease in desired dilution occurs, it can be achieved by quickly eliminating water injection at the cylinder.
As such, to address knock issues, the amount of water direct injected may be maintained above a lower threshold and below an upper threshold. Thus, in one example, as the EGR is transitioned in, the amount of water direct injected may be reduced up to the lower threshold. Thereafter, further EGR transients may be, at least temporarily, addressed with some VCT retard. In another example, as the EGR is transitioned in, the amount of water direct injected may be increased up to the upper threshold. Thereafter, further EGR transients may be, at least temporarily, addressed with some VCT advance. In alternate embodiments, an amount of boost, a throttle adjustment, and/or an amount of spark ignition timing advance may be used to compensate for dilution and torque transients.
It will be appreciated that while the depicted example is illustrated with reference to water as the direct injected knock control fluid, in alternate examples, such as where the injected knock control fluid is an alcohol blend, the amount and timing of the direct injection may be adjusted based on the type of the injected fluid. Specifically, the amount may be based on a combination of the inherent octane effect, dilution effect, and charge cooling effect of the injected knock control fluid. For example, where the injected fluid has a high dilution effect, a higher amount of fluid may be injected to compensate for EGR transients. In another example, where the injected fluid has little dilution effect but has high charge cooling and/or octane effect, a lower amount of fluid may be injected to control knock, and none may be injected to compensate for EGR transients.
In one example, the combination of effects may be inferred from the molar or volumetric composition of the injected fluid. For example, where the injected fluid is a blend including an alcohol fuel, the molar composition may be based on the volumetric fractions of the constituent fluids in the fuel blend, as well as their molecular weights and densities. Thus, as the alcohol content of the injected fluid increases, the inherent octane effect and charge cooling effects increase so direct injection amount for knock control may be decreased. Similarly, as the water content of the injected fluid increases, the dilution effect increases so the direct injection amount for dilution may be increased, and a smaller subsequent amount of VCT, and/or EGR dilution may be required.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.